The present invention generally relates to optical devices for use in optical telecommunications and more particularly to an optical waveguide that has a variable refractive index.
In the optical telecommunication systems, the control of refractive index of various optical elements is a key technique. For example, one can switch the path of optical signals by changing the refractive index of the optical waveguide. By using an optical waveguide capable of changing the refractive index for the laser diodes, on the other hand, one can modify the effective resonant optical length of the laser diode and hence the oscillation wavelength. Further, the optical waveguide having the variable refractive index can be used, in combination with a laser diode, to construct an optically bistable laser diode. Such an optically bistable laser diode is an essential device for constructing a digital optical processing system.
In the semiconductor waveguides, the refractive index can be changed either by applying a control voltage or by injecting a control current. When a control voltage is applied, an electric field is induced in the semiconductor waveguide and such an electric field in turn causes the desired refractive index change by inducing the Franz-Keldysh effect or Quantum Confinement Stark effect. Generally, the change of the refractive index achieved by such an electric field effect is small and one needs a large control voltage to achieve a necessary refractive index change.
The injection of carriers, on the other hand, provides a large refractive index change by causing the band filling effect or plasma effect, and is expected to become an important as sell as fundamental process for controlling the tunable laser diodes or tunable filters in the wavelength multiplex optical networks or for constructing the optical switches that switches the path of the optical beam. In these applications, it is essential to have a large refractive index change with a small injection current.
FIG. 1(A) sows the refractive index change profile achieved in a bulk semiconductor material, wherein the horizontal axis represents an energy E while the quantity .rho. on the vertical axis represents the density of state of the carriers. As is well known, the density of state .rho. changes parabolically with the energy E in the bulk crystal, and the carriers fill the states starting from the bottom edge of the hand in accordance with the Fermi-Dirac distribution function as shown by the shaded area of FIG. 1(A). In FIG. 1(A), the shaded area represents the states filled by the electrons. In response to the filling of the electrons, there appears a profile of refractive index .DELTA.n as plotted also in FIG. 1(A). As will be understood from the description below, the refractive index profile .DELTA.n includes the contributions of the electrons that are distributed over a wide energy spectrum in correspondence to the shaded area, and the magnitude of .DELTA.n becomes inevitably small as a result of broadening caused by these contributions.
FIG. 1(B) shows the density of state that is achieved in the quantum structure wherein the carriers are confined three-dimensionally in a minute region or a quantum well box. In such a case, the density of state .rho. is represented approximately by the Dirac's .delta.-function as illustrated, and one obtains a corresponding distribution of the refractive index .DELTA.n according to the Kramers-Kronig relation, which describes a correspondence between the real part and the imaginary part of a physical quantity based upon the causality, as illustrated also in FIG. 1(B).
In FIG. 1(B), it should be noted that the curve .DELTA.n represents the contribution from the .delta.-function-like state density .rho. located at the energy E.sub.O and has a magnitude much larger than the magnitude of the refractive index change achieved in the bulk crystal. It should be noted further that the curve .DELTA.n of FIG. 1(A) is obtained as a result of the superposition of the curve .DELTA.n of FIG. 1(B) for the carriers of different energies. Thereby, the refractive index change .DELTA.n becomes substantially broad and small in the bulk crystal as a result of the superposition.
In view point of realizing a large refractive index change in the semiconductor waveguides, it is advantageous to device a structure that shows the density of state similar to FIG. 1(B). For this purpose, the inventors of the present invention have previously proposed, in the Japanese Laid-open Patent Application 3-235915 published on Oct. 21, 1991, the use of a multiple quantum well (MQW) structure for the semiconductor waveguide.
FIG. 2(A) shows the band structure of the MQW waveguide proposed in the foregoing prior art.
Referring to FIG. 2(A), the MQW waveguide is formed as an alternate deposition of a quantum well layer 1 and a barrier layer 2 wherein the barrier layer 2 has a band gap much larger than the band gap of the quantum well layer. Thereby, the barrier layer 2 acts as a potential barrier and confines the carriers vertically into the quantum well layer 1 formed between the barrier layers 2.
In such a MQW structure, it is well known that discrete quantum levels Ec.sub.1 and Ev.sub.1 appear in the conduction band E.sub.c and the valence band E.sub.v when the thickness L.sub.w of the quantum well layer 1 is decreased approximately below the de Broglie wavelength of the carriers. With further decrease in the thickness L.sub.w, the quantum levels Ec.sub.1 and Ev.sub.1 increase as indicated by arrows, resulting in an increased energetical separation E.sub.1 between the quantum level Ec.sub.1 and the quantum level Ev.sub.1.
FIG. 2(B) shows the density of state for the MQW structure of FIG. 2(A). Generally, the density of state for this case is represented by a step-like pattern, wherein the step pattern designated as .rho..sub.1 corresponds to the case where the thickness L.sub.w is set at a first value L.sub.1, the step pattern designated as .rho..sub.2 corresponds to the case where the thickness L.sub.w is set at a second value L.sub.2, and the step pattern designated as .rho..sub.3 corresponds to the case where the thickness L.sub.w is set at a third value L.sub.3. It will be noted that the step height increases significantly with decreasing thickness L.sub.w of the quantum well layer 3 as a result of the significant increase in the number of available quantum states, which in turn is caused as a result of the shift of the quantum level in the higher energy side.
Similarly to the case of the bulk crystal shown in FIG. 1(A). It will be noted that the carriers fill the quantum sates starting from the bottom edge of the step in accordance with the Fermi-Dirac distribution function. Thereby, the carriers distribute in the quantum level as represented in FIG. 2(B) by the shading.
In the diagram of FIG. 2(B), it should be noted that the number of carriers that occupy the quantum level remains constant even when the thickness L.sub.w of the quantum well layer 1 is increased from L.sub.1 to L.sub.3. Thereby, the shaded area remains constant in the case where L.sub.w is set equal to L.sub.1 and in the case where L.sub.w is set equal to L.sub.3. This means that the range of the energy levels that the carriers occupy is substantially reduced by decreasing the thickness L.sub.w, and the density of state approaches to the .delta.-function-like pattern shown in FIG. 1(B). Thereby, one can increase the magnitude of the refractive index change .DELTA.n.
As the profile of the refractive index change .DELTA.n disappears when there is no carrier in the quantum levels Ec.sub.1 and Ev.sub.1, one can control the refractive index of the quantum well layer 1 by injecting or removing the carriers. Thus, the optical waveguide of this prior art is suitable for maximizing the range of change of the refractive index.
In this conventional approach, however, there exists a problem in that there is formed a band tail at the bottom edge of the step as shown in FIG. 3 in the real MQW structure due to the scattering of the carriers by the impurities or phonons. There, the band tail extends in the lower energy side typically by 50.+-.15 meV from the energy E.sub.1 that corresponds to the lower edge of the ideal optical absorption band. It should be noted that the density of state shown in FIG. 2(B) is for the ideal case where the effect of such scattering is not considered. As the band tail is formed at the bottom edge of the band, the carriers inevitably occupy the states corresponding to the band tail part and these carriers cause an absorption of the optical beam whenever the optical beam is supplied to the waveguide with the wavelength corresponding to the band tail. When the wavelength of the input optical beam is determined by the requirement of the optical transmission path etc., it is necessary to construct the MQW waveguide such that the unwanted absorption of the optical beam by the band tail does not occur while maintaining a maximum refractive index change.